When using video conferencing it is useful to enable participants in the conference to transmit text or other data simultaneously with the transmission of the video (i.e. the audio and image components of the conference). This may be of use, for example, if the purpose of the conference is to give a presentation. In the case of a presentation the data could be a slide show or photographs. Alternatively, the data may be formulae in mathematical proofs or any other data which may need to be modified in real time as the conference or presentation progresses.
One method of transmitting data is to use two separate video channels connecting two endpoints either directly or indirectly through a multi conference unit (MCU). This may be achieved using the ITU-T H.239 format that was developed to enable the use of two video channels simultaneously within a single video conference.
The two video channels may be configured in order to transmit different data types. For example, a first video channel may be adapted to transmit video data which is which is advantageously displayed at a high frame rate in order that movements can be seen smoothly. In order to avoid using a large amount of network bandwidth, data sent using a high frame rate is usually sent at a low resolution. The second video channel may be adapted to transmit presentation data which, because it can be shown at a low frame rate without detracting from the appearance of the data, can be displayed at a high resolution so that all the detail of the presentation data can readily be seen.
Video data is defined for the purposes of this description as data having an audio component, such as the voice of a person, and an image component such as the view of the person. Presentation data is defined for the purposes of this description as being data which is preferably displayed using a low frame rate but high resolution. It may be, for example, text, one or more PowerPoint slides or pages from a pdf file.
FIG. 1 illustrates a prior art configuration of a video conference that is being transmitted using H.239. It can be seen that the video conference is taking place between two endpoints 10, 12 that are H.239 enabled i.e. are able to display data transmitted using H.239 and transmit data using H.239.
Each of the H.239 enabled endpoints 10, 12 has two channels 14, 16 along which data is transmitted to an MCU 18. The two channels may both transmit video data or presentation data. Alternatively, one channel may transmit video data and the other presentation data. The MCU 18 will process the data as required and transmit the processed data to the relevant endpoint 10, 12. Data may also be transmitted directly between the two H.239 enabled endpoints.
The user may view the data either on separate screens; one for presentation data and one for the video data, or, alternatively, the user may view the data as a composite image on a single screen of an H.239 enabled endpoint. H.239 can therefore be used in a video conference to enable data received from two inputs such as a presentation input for presentation data and a video input to be output onto one screen at the endpoint.
In a conventional H.239 conference a token is created which is passed between conference participants. Only the participant in the conference holding the token can transmit presentation data to all the other participants. This means that there is no way for the conference participants to interact directly with any presentation data displayed unless they are holding the token. This restricts the interaction of the participants.